As a method for producing a hard disk substrate, there has been performed a method that includes applying electroless NiP plating to an aluminum or aluminum alloy substrate, which has been mechanically processed, to form a plating film on the surface of the substrate so that the plating film is used as a base of a magnetic film (see Patent Literature 1).
Herein, in order to achieve high recording density of a hard disk recording device, it is necessary to set the flying height of a recording/reading head as low as possible. Thus, after a plating film is formed through electroless NiP plating, a polishing step of smoothing the surface of the plating film is performed by polishing the surface with free abrasive grains.